According to increasing requirement for high integration and high density of a semiconductor device, a photolithography technology for realizing a high resolution has been studied and developed in order to form a more minute pattern on a wafer. The resolution R of an exposure apparatus is defined by Equation 1, in which k1 is a fixed number corresponding to a process, λ is the wavelength of exposure light, and NA is the number of lens apertures in the exposure apparatus;
                    R        =                  k          ⁢                                          ⁢          1          ⁢                      λ            NA                                      1      
As shown in Equation 1, in order to achieve a high resolution, the number NA of the lens apertures in the exposure apparatus must be increased, and the wavelength of the exposure light must be decreased. Accordingly, the wavelength of the exposure light practically used for the exposure apparatus has been gradually decreased from I-ray (365 nm) to a KrF Excimer laser (248 nm), an ArF Excimer laser (193 nm), an F2 laser (157 nm), and the like.
FIG. 1 shows a mask having a storage node contact pattern according to a conventional technology using the KrF laser. As shown in FIG. 1, when using the KrF laser, if sizes of the storage node contacts on the mask are 93 nm, 90 nm and 87 mm, critical dimensions of simulation contacts become 103.7 nm, 95.0 nm and 86.1 nm, respectively. A mask error factor in this case is about 11.8, which causes drastic changes in dimensions of a pattern on a wafer due to slight changes in dimensions of a pattern on the mask. Accordingly, considering that an acceptable mask error factor for mass production is less than 5, it can be appreciated that a mask error factor of 11.8 is remarkably high.
FIG. 2 shows a storage node contact pattern exposed on a wafer by the conventional technology. As shown in FIG. 2, when a storage node contact pattern having a half pitch of 90 nm on a 6% half tone mask is exposed to a wafer by means of a 0.80 NA KrF exposure apparatus using the mask, there occurs a defect “b” wherein a pattern unit of the minute storage node contact pattern is not regularly opened on the wafer or a defect “c” wherein the pattern units are bridged to each other.
FIG. 3a shows an irregular pattern caused by an error on the mask having the storage node contact pattern according to the conventional technology, and FIG. 3a shows a defective pattern on the wafer caused thereby.
As shown in FIGS. 3a and 3b, it can be appreciated that there occurs a defect wherein a pattern unit of the storage node contact pattern is not regularly opened on the wafer due to the error on the mask caused by repetitious arrangement of the storage node contact pattern.
As such, according to the conventional technology, there is a problem in that, when using the KrF laser, the critical dimension of the minute pattern, such as a minute storage node contact pattern, is not realized with high precision.